The invention is concerned with a glazing comprising heating wires.
It is well known in the field of automotive windscreens to provide an array of fine, closely spaced heating wires embedded in an interlayer material between two outer plies, usually of glass.
The heating wires extend between and are in electrical contact with two opposing busbars, typically positioned adjacent to upper and lower edges of the glazing.
EP0385785 and EP0385791 disclose a busbar which comprises a narrow metal strip and a wider metal strip, such that the heating wires are arranged between the metal strips. In a process for manufacture, a narrow metal strip is laid over a sheet of interlayer material. A heated iron is run over the narrow metal strip, to melt the interlayer material locally and embed the narrow metal strip in the interlayer material. The interlayer material is placed on a drum and wires are then laid over the interlayer material and pressed into it by a heated presser roller (as disclosed in US2009/0206191). The interlayer material is then laid on a flat table and a wider metal strip is applied over the narrow metal strip so that it extends beyond the narrow metal strip on both edges thereof. A hot iron is then run over the wider metal strip, to bond it to the interlayer material, and to secure the ends of the wires, by trapping them between the metal strips and the interlayer. The interlayer material is then arranged between first and second glass plies and the resulting assembly is heated in an autoclave in known manner. In the autoclave, the interlayer softens so that the heating wires and metal strips become fully embedded in the interlayer and thus securely fixed in position.
To improve an electrical connection between the busbar assembly and the heating wires, the wider metal strip may be provided with a low melting point solder layer on part of a surface in contact with the heating wires. Low melting point solder is defined as solder which melts and fills spaces between the heating wires in an autoclave, i.e. having a melting point not more than 150 degrees C., the maximum autoclave temperature. On solidification, the low melting point solder bonds the heating wires firmly in position and provides a durable electrical connection between the wider metal strip and the heating wires. A similar low melting point solder layer may be applied to the narrow metal strip.
EP1110431 discloses that products manufactured by the process disclosed in EP0385791 have lower in-service failure rates. The improvement is thought to be due to (a) low melting point solder holding the heating wires, so that heating wires do not lose contact with a busbar and (b) low melting point solder filling spaces between heating wires, so that moisture is prevented from entering a busbar assembly and thus a possibility of an electrolytic reaction between dissimilar metals is avoided.
JP2000-030847 discloses a glazing in which adhesion between busbars and heating wires is improved and good conduction maintained over a long period of time by applying a conductive adhesive between the busbars and the heating wires. The conductive adhesive may be a solder layer, an acrylic hardening glue or an epoxy system heat cure glue. At least one busbar is embedded in interlayer material. The glazing has improved performance in a peel test and in a 30 day thermal cycle test.
WO2011023974 discloses a glazing comprising a conductive pathway for heating the glazing and a busbar. A busbar may be glued to a ply of glazing material to obtain a good mechanical interface. A busbar may be coated with low melting point solder or conductive adhesive to provide a good electrical connection to the conductive pathway.
U.S. Pat. No. 6,396,026B2 discloses a glazing comprising a busbar on a thermoplastic layer, the busbar comprising a lower copper foil strip which is fixed to the thermoplastic layer using an adhesive. Heating wires are laid on the lower copper foil strip and an upper copper foil strip is laid on the heating wires. The lower copper foil strip has a previously applied layer of soft solder as a tin coating on the freely accessible side. A soldering operation is performed by applying a soldering iron to the upper copper foil strip.
There remains a need for an alternative glazing, comprising heating wires, having a low in-service failure rate and a corresponding process for manufacture, avoiding at least one process step needing a heated iron, would be desirable.